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| 1 /* |
| 2 * Copyright 2015 Google Inc. |
| 3 * |
| 4 * Use of this source code is governed by a BSD-style license that can be |
| 5 * found in the LICENSE file. |
| 6 */ |
| 7 |
| 8 #ifndef SkSemaphore_DEFINED |
| 9 #define SkSemaphore_DEFINED |
| 10 |
| 11 #include "SkTypes.h" |
| 12 #include "SkAtomics.h" |
| 13 |
| 14 /** |
| 15 * SkSemaphore is a fast mostly-user-space semaphore. |
| 16 * |
| 17 * A semaphore is logically an atomic integer with a few special properties: |
| 18 * - The integer always starts at 0. |
| 19 * - You can only increment or decrement it, never read or write it. |
| 20 * - Increment is spelled 'signal()'; decrement is spelled 'wait()'. |
| 21 * - If a call to wait() decrements the counter to <= 0, |
| 22 * the calling thread sleeps until another thread signal()s it back above 0. |
| 23 */ |
| 24 class SkSemaphore : SkNoncopyable { |
| 25 public: |
| 26 // Initializes the counter to 0. |
| 27 // (Though all current implementations could start from an arbitrary value.) |
| 28 SkSemaphore(); |
| 29 ~SkSemaphore(); |
| 30 |
| 31 // Increment the counter N times. |
| 32 // Generally it's better to call signal(N) instead of signal() N times. |
| 33 void signal(int N = 1); |
| 34 |
| 35 // Decrement the counter by 1, |
| 36 // then if the counter is <= 0, sleep this thread until the counter is > 0. |
| 37 void wait(); |
| 38 |
| 39 private: |
| 40 // This implementation follows the general strategy of |
| 41 // 'A Lightweight Semaphore with Partial Spinning' |
| 42 // found here |
| 43 // http://preshing.com/20150316/semaphores-are-surprisingly-versatile/ |
| 44 // That article (and entire blog) are very much worth reading. |
| 45 // |
| 46 // We wrap an OS-provided semaphore with a user-space atomic counter that |
| 47 // lets us avoid interacting with the OS semaphore unless strictly required: |
| 48 // moving the count from >0 to <=0 or vice-versa, i.e. sleeping or waking th
reads. |
| 49 struct OSSemaphore; |
| 50 |
| 51 SkAtomic<int> fCount; |
| 52 OSSemaphore* fOSSemaphore; |
| 53 }; |
| 54 |
| 55 #endif//SkSemaphore_DEFINED |
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